In several diesel engines today, hydraulically-actuated devices, such as hydraulically-actuated fuel injectors and engine brakes, are controlled by electrically-actuated fluid control valves. Depending on the positioning of a valve member, the fluid control valve either connects the hydraulic device to a source of high pressure actuation fluid causing the device to activate, or connects the hydraulic device to a low pressure actuation reservoir causing the device to deactivate, reset itself, or remain inactive. The movement of the valve member is controlled by an electrical actuator, such as a solenoid or piezo actuator. For instance, hydraulically actuated fuel injectors such as that shown in U.S. Pat. No. 5,738,075 issued to Chen et al. on Apr. 14, 1998, include a solenoid driven fluid control valve that is attached to an injector body.
Typically, in order to connect the hydraulic device to the source of high pressure, electric current is supplied to the electrical actuator to move the valve member against the bias of a spring. However, over the years, engineers have found that a pressure differential across the fluid control valve can affect the ability of the valve to operate in a predictable manner. The pressure differential across the fluid control valve can cause the velocity of the fluid to increase and the pressure to decrease, especially in the region around a valve seat. These changes within the pressure and velocity of the fluid can create flow forces that act against the movement of the valve member. Thus, the electrical actuator must move the valve member not only against the bias of the spring but also against the flow forces. These flow forces generally increase as the pressure differential across the valve increases. Engineers design the hydraulic system such that the voltage available to the electrical actuator is sufficient to move the valve member from its closed position toward its open position against the bias of the spring and the flow forces at the highest expected pressure differentials, which corresponds to the highest expected rail pressure in the case of a fuel injection system.
While the method of using electrically-actuated fluid control valves in order to control hydraulically-actuated devices has performed well, there is room for improvement. For instance, federal regulations require that most vehicles and machinery be able to operate within a range of voltage, such as 9–16 volts. Thus, engineers are constantly searching for strategies to operate electronically controlled engine components, such as fuel injectors or engine brakes, at the lower end of this voltage range. Further, when voltage (energy) available to the electrical actuator decreases, possibly due to a problem within the electrical circuitry or power supply of the vehicle or machinery, the electronic control module may be unable to provide sufficient electric current to the electrical actuator in order to move the valve member to, and hold the valve member in, its open position at the higher rail pressures. Thus, when the voltage falls below a certain level, the fluid control valve is unable to sufficiently fluidly connect the fuel injector to the source of high pressure actuation fluid and activate the fuel injector in a predictable manner. In other words, the valve may behave erratically, or not at all. The result being that fuel cannot adequately and/or accurately be injected into the engine and the vehicle or machinery will then stall and/or misfire. This can lead to towing expenses and other lost productivity and inconveniences. Moreover, if the electrical problem causing the voltage to decrease occurred at a time when the engine brake is needed in order to slow the vehicle or machinery, such as when descending a steep hill, the engine brake may not operate properly, potentially resulting in a run-away vehicle.
The present invention is directed at overcoming one or more of the problems set forth above.